Two-mode Dynamics of Vertical-Cavity Surface-Emitting Lasers
Résumé
We first report on recently experimentally discovered two-mode dynamics in solitary Vertical-Cavity Surface-Emitting Lasers (VCSELs), which manifests the microscopic spin-flip processes taking place in the semiconductor quantum dots (QD)[1] We demonstrate that such dynamics leads to polarization chaos, a new phenomenon for class B semiconductor lasers. Our QD-VCSEL emits linearly polarized light at threshold and undergoes a pitchfork bifurcation when increasing injection current that creates two symmetrical elliptically polarized solutions ( Fig. 1a). Then, a sequence of bifurcations (Fig. 1b) brings it to two single-scroll chaotic attractors oscillating around the now unstable elliptically polarized steady-states (Fig.1c). Finally, the two chaotic attractors merge into a double-scroll "butterfly" chaotic attractor (Fig.1d). Experimentally, this "butterfly" attractor is observed as a mode hopping between two elliptically polarized modes (Fig.1e,f) [2]. The dwell time, i.e. the average time between two successive jumps exponentially decreases with injection current which is opposite to noise-induced polarization mode hopping in quantum well VCSELs [3]. Next, we review the features of VCSEL nonlinear dynamics induced by optical injection and optical feedback that are due to the lack of polarization anisotropy and to multiple transverse mode operation. In case of optical injection we demonstrate a new Hopf bifurcation on a two-polarization-mode solution that delimits the injection locking region and a new resonance tongue for large positive detunings that is due to polarization switching and injection locking of first-order transverse mode [4]. For the case of optical feedback in VCSELs[5] we show that for long and short external cavities the Lang-Kobayashi model is in good agreement with experiments on polarization switching, mode hopping and antiphase oscillatory dynamics at the delay time. These last dynamics modify polarization residence-time distribution making it oscillatory. We demonstrate numerically and experimentally coherence resonance in such bistable, time-delayed optical system. We present results on low frequency fluctuation and pulse-package dynamics in VCSELs and discuss the role of light polarization.